In epitaxial growth of semiconductor structures and devices, a substrate is needed on which the growth of the semiconductor material is initiated, and which serves as a support for the grown semiconductor layers. The properties of the substrate play a key role in the quality of the epitaxially grown semiconductor layers. For example, a lattice mismatch between the substrate and the semiconductor material epitaxially grown thereon causes stresses and can result in formation of dislocations in the semiconductor material. The dislocations can significantly deteriorate the performance of a semiconductor device, e.g. a light emitting diode (LED), formed on the substrate. Additionally, a difference in the thermal expansion coefficient between the substrate and the epitaxially grown layers can induce stresses in the semiconductor layers. Finally, a substrate can be used for efficient thermal management of the device provided that the substrate has an overall low thermal resistance. Thus, most preferably, the substrate should be formed of the same material to be grown epitaxially thereon. A situation in which the substrate and the epitaxially grown layers are of the same material is referred to as homoepitaxy.
Unfortunately, some widely used compound semiconductor materials such as, for example, gallium nitride (GaN) and other group III nitrides, have significant problems in their bulk fabrication. Production of bulk material in the form of single-crystal wafers can be so challenging and expensive that it is not suitable for industrial-scale manufacturing. In such cases, heterosubstrates (also known as foreign substrates), i.e., substrates formed of a material different from the material to be epitaxially grown thereon, must be used. However, despite carefully optimizing the substrate material for the actual semiconductor material to be grown, the adverse effects of crystal lattice mismatch and difference in thermal expansion between the heterosubstrate and the material to be grown thereon are difficult, if not impossible, to eliminate entirely. To avoid these undesirable effects, different kinds of semiconductor templates have been developed for epitaxial growth of different materials. Templates are typically multi-layered epitaxial structures adjusted to the heterosubstrate with a cap layer made of a material optimized for later epitaxial growth of the semiconductor device structures. By using such templates, e.g., a GaN device can be grown on a sapphire substrate. Such a template improves the quality of the epitaxial layers but is still not able to sufficiently suppress generation of thermo-mechanical stresses.